An ink-jet printer is a type of non-impact printer which forms characters and other images by controllably spraying drops of ink from a print head. The print head ejects liquid ink through multiple nozzles in the form of annular drops which travel across a small air gap and land on a recording media. The drops are very small as ink-jet printers commonly print within a range of 180 to 600 dots per inch (dpi). The ink drops dry shortly thereafter to form in combination the desired printed images.
One problem associated with ink-jet printers concerns the amount of ink deposited from the print head during the formation of each drop. The quantity of deposited ink, commonly referred to as the "drop-volume" of the print head, is dependent on the temperature of the print head. If the print head is cool, it will deposit less ink in each droplet. Low drop-volume results in poor quality images that appear faint or washed out. Conversely, if the print head is too hot, it will eject more ink in each droplet. High drop-volume increases the amount of time necessary for the image to dry and can yield poor quality images that appear too dark or have poor resolution. Accordingly, it would be desirable to have the print head deposit an opt/mum drop-volume while at its preferred operating temperature.
However, another printing dynamic is also involved. As a print head is printing, its temperature will gradually rise from an initial temperature to a steady-state operating temperature. As the print head heats up, the amount of ink that is deposited will likewise gradually rise. Another complicating factor is that the print head temperature can fluctuate during each pass of the print head over a recording media. The print head cools down during the non-printing time between the end of the previous line and the start of the next line. The print head then warms up again while printing the next line.
The changing print head temperature makes it difficult to deposit a uniform and optimum amount of ink. If the nominal drop-volume of the print head is set to yield the desired print quality and dry times when the print head is cool, the print head will deposit too much ink on the recording media when it warms up. On the other hand, if the nominal drop-volume of the print head is calibrated so that the print quality and dry times are good when the print head is warm, the print head will eject too little ink causing faint images at the start of the line when the print head is cool. It is therefore desirable to dynamically control the print head temperature to produce a more uniform drop-volume over the printing cycle.
One approach to controlling the print head temperature is described in U.S. Pat. No. 4,910,528, which is assigned to Hewlett-Packard Company. The control system disclosed in this patent maintains the temperature of the print head within an acceptable operating range by measuring the current temperature, predicting the heat loading on a subsequent pass over the recording media, and then adjusting the temperature of the print head. Temperature adjustment can be made by either heating individual ejectors or modifying operation of the printer to permit cooling of the ejectors. Heating is accomplished by applying a low level current to ejector resistors. Cooling is achieved by slowing the printing rate for one or more passes to cool the print head.
Another thermal control system is described in U.S. Pat. No. 5,107,276. To prevent temperature fluctuations, the nozzles of the print head that are not being used to eject ink droplets are selectively energized with energy pulses having insufficient magnitude to vaporize the ink. In this manner, the low energy pulses simply warm the print head without ejecting ink drops. The number of low energy pulses used to warm up the print head is determined by counting the number of ink drops fired in a given time period. A microprocessor is employed for this task. From this drop count, the number of low energy pulses necessary to maintain the print head temperature at the prescribed level is determined via a look-up table. The low energy pulses are then applied to the print head. The ambient or print head temperature may optionally be measured and used to adjust the number of compensating pulses stored in the look-up table. According to this count/look-up/adjust technique, the print head temperature is controlled without having to measure the temperature directly.
This invention is an improvement over the control systems described in U.S. Pat. Nos. 4,910,528 and 5,107,276. This invention provides an effective print head temperature control circuit which is more simple, and thus less costly to implement, than the comparatively more complex systems in these patents.